US President Barack Obama launched an intense lobbying effort on Sunday to sway sceptical lawmakers weighing whether to support a military strike against Syria.

The strategy, outlined by a senior White House official, came as Secretary of State John Kerry said Washington had proof Damascus used sarin gas in a deadly attack August 21.

Arab League foreign ministers, meanwhile, urged the United Nations and international community to take “deterrent” action, while blaming the regime for the alleged chemical weapons assault.

All the while, a defiant President Bashar al-Assad insisted that Syria would confront any military action.

With all eyes on Washington, Obama – as well as Vice President Joe Biden and the White House chief of staff – made individual calls to members of Congress on Sunday, according to the official. More are planned for Monday, a federal holiday.

That came on top of a classified briefing held on Capitol Hill Sunday afternoon. More meetings, at the White House, are planned for select members of Congress over the coming two days.

On the heels of a surprise announcement by the US leader, the White House on Saturday formally asked Congress for authorisation to conduct strikes in a draft resolution framing a narrow set of operations.

But it remains to be seen if war-weary lawmakers will endorse Obama’s push for action – or hand him a bitter defeat. Many, including influential Republican senator John McCain, remain unsure they can support the White House resolution.

“In all calls and briefings, we will be making the same fundamental case: the failure to take action against Assad unravels the deterrent impact of the international norm against chemical weapons use, and it risks emboldening Assad and his key allies – Hezbollah and Iran – who will see that there are no consequences for such a flagrant violation of an international norm,” the senior White House official said.

“Anyone who is concerned about Iran and its efforts in the region should support this action.”

Pushing the administration’s argument for limited military action, Kerry told US television talk shows that hair and blood samples given to the US from emergency workers on the scene of the August 21 attacks showed signs of the powerful sarin nerve gas.

A key US ally, however, suggested that limited strikes were not the way to go.

Turkish Prime Minister Recep Tayyip Erdogan, who favours major international military intervention in Syria, stressed on Sunday that limited strikes would only exacerbate the situation.

“Targeted strikes would not only fail to lead us towards a solution but would make conditions even more difficult in Syria,” he told a meeting in Istanbul.

He added that Assad should “immediately leave office and go to a country willing to take him”.

Turkey shares a long border with Syria.

Video: Kerry: Sarin gas found in Syria samples

Meeting in Cairo, meanwhile, Arab League foreign ministers issued a statement calling upon the United Nations and international community “to assume their responsibilities in line with the UN Charter and international law by taking the necessary deterrent measures.”

Syria’s opposition National Coalition had earlier pleaded with the ministers to back a US-led strike, while Saudi Foreign Minister Saud al-Faisal said failure to do so would bolster Assad’s forces to “pursue its crimes”.

But Assad, whose regime has faced an uprising since March 2011 that a watchdog says has cost 110,000 lives, came out swinging.

“Syria ... is capable of facing up to any external aggression just as it faces up to internal aggression every day, in the form of terrorist groups and those that support them,” SANA news agency quoted him as saying.

Assad’s comments were his first since Obama’s move Saturday.

On the streets of Damascus, residents were scornful of the president’s decision to back down from immediate action.

“He who talks a lot doesn’t act,” said Souad, mocking the US leader as a ”coward” for delaying the decision.

“Obama is a coward. He didn’t strike because he knows that our President Bashar (al-Assad) is all-powerful,” said the employee of nationality electricity firm Ferdaws, in the northeast of the capital.

On Saturday, Obama said the US military was poised to react at any time.

The US Navy has deployed an amphibious transport ship to the Mediterranean, where five destroyers are already in place for possible missile strikes on Syria, a defence official said on Sunday.